10. Watchmen
What Was It: Watchmen's film rights were snapped up almost immediately after the comic's release by 20th Century Fox and producer Joel Silver. After Alan Moore declined an offer to write the screenplay himself, Sam Hamm was brought on and had a lot of difficulty condensing it into a 128-page script. Later, Terry Gilliam was attached as director and had Charles McKeown rewrite the script to restore scenes from the comic that Hamm had excised. Gilliam's script would have radically altered the ending, with Ozymandias convincing Dr. Manhattan that his very existence is what changed the whole balance of the world, and so Ozymandias was able to convince Dr. Manhattan to go back in time and prevent his transformation. As a result, the movie would have ended with Nite Owl, Silk Spectre and Rorschach standing in Times Square and a kid coming up to them and saying, "hey, you're just like in my comic book." Basically, by preventing his own transformation, Dr. Manhattan would have changed the world of Watchmen into a comic book.
What Happened: As both Silver and Gilliam's previous films went over budget, they were only able to raise a quarter of what they needed for Watchmen. Gilliam abandoned the project and later claimed it was unfilmable. Gilliam was later invited back to the project, but refused unless it could be done as a five-hour miniseries.